"The image, called the Hubble Deep Field (HDF), was assembled from 342 separate exposures taken with the Wide Field and Planetary Camera 2 (WFPC2) for ten consecutive days between December 18 and 28, 1995.
Representing a narrow "keyhole" view stretching to the visible horizon of the universe, the HDF image covers a speck of the sky only about the width of a dime located 75 feet away. Though the field is a very small sample of the heavens, it is considered representative of the typical distribution of galaxies in space because the universe, statistically, looks largely the same in all directions. Gazing into this small field, Hubble uncovered a bewildering assortment of at least 1,500 galaxies at various stages of evolution.
Nearly a year of preparation preceded the observation. The HDF team selected a piece of sky near the handle of the Big Dipper (part of the northern circumpolar constellation Ursa Major, the Great Bear). The field is far from the plane of our Galaxy and so is "uncluttered" of nearby objects, such as foreground stars. The field provides a "peephole" out of the galaxy that allows for a clear view all the way to the horizon of the universe.
Test exposures made in early 1995 with Hubble and the 4-meter telescope at Kitt Peak National Observatory also confirmed the field is devoid of large galaxy clusters, which would interfere with seeing farther and fainter objects. The target field is, by necessity, in the continuous viewing zone (CVZ) of Hubble's orbit, a special region where Hubble can view the sky without being blocked by Earth or interference from the Sun or Moon.
The most common type of galaxy in the universe are (or 'were') small irregular objects called blue dwarfs. However, they were edited out.
Produced by astronomers at the State University of New York at Stony Brook."